Wireless communication is playing an evermore important role in today's world. More people are signing up for wireless service, and they are expecting their wireless terminals (e.g., pagers, phones, faxes and computers) to perform more functions more rapidly and efficiently than ever before.
As wireless terminals and their associated networks grow in size and complexity, communication standards that give rise to interoperability among various manufacturers' systems become vital. Standards bodies are responding by designing new wireless networks, protocols and terminal features in a multinational effort called “3G,” which is short for “third generation wireless information technology.”
One standard that is garnering much attention these days is the “Universal Mobile Telecommunications System” standard, or UMTS. UMTS calls for the use of WCDMA (which stands for Wideband Code-Division Multiple Access) as its radio interface technology. WCDMA is the preferred technology, because its 5 Mhz carrier bandwidth can accommodate very high data communication speeds and therefore the speed, flexibility and features that people are beginning to demand.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the “TIA/EIA-98-D Recommended Minimum Standard for Dual-Mode Wideband Spread Spectrum Cellular Mobile Station” (the IS-98 standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213 and 3G TS 25.214 (the WCDMA standard) and (4) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents that includes Nos. C.S0002-A, C.S0005-A, C.S0010-A, C.S0011-A, C.S0024 and C.S0026 (the CDMA2000 standard). These standards are incorporated herein by reference.
WCDMA can support more than one overlying transport format. In fact, UMTS currently supports 16 such transport formats for WCDMA. Each format calls for bits of information transmitted between stations to be gathered into frames and given Cyclic Redundancy Check (CRC) bits to ensure their integrity. However, the similarity between the various transport formats ends there. For a UMTS-compliant receiver to be able to interpret frames received from a UMTS-compliant transmitter, the receiver must come to know which one of the 16 possible transport formats the transmitter used to format the frames.
Two ways exist for the receiver to gain such knowledge under the current UMTS standard. As a first alternative, the transmitter could expressly instruct the receiver as to which transport format it is using. UMTS accommodates an optional Transport Format Indicator (TFI) for just this purpose. As a second alternative, the receiver could determine the transport format for itself by, for example, looking for valid CRC bits in the frames. This process is called Blind Transport Format Detection, or BTFD.
The apparent advantage in the first alternative is that the receiver's time and effort (which translates to energy consumption) are saved. However, bandwidth which could otherwise be used to carry user data is instead forced to carry TFIs. BTFD allows that bandwidth to be preserved for carrying user data, but obviously imposes processing effort and concomitant energy consumption on the receiver.
Before venturing into a specific description of the nature of the processing effort, one technical detail should be discussed. WCDMA receivers do not directly receive bits from WCDMA transmitters. Instead, WCDMA receivers receive symbols; each symbol conventionally represents more than one bit in a given frame. Once received, the symbols are decoded, often in a circuit called a “Viterbi decoder.” A Viterbi decoder decodes convolutionally encoded bits using the Viterbi algorithm and is known to those skilled in the art.
Currently, BTFD is carried out by first allowing the Viterbi decoder to decode received symbols into bits. A frame's worth of bits generated by the Viterbi decoder is then collected and stored in a buffer. Then, a software-based iterative process is initiated, whereby the frame is tested under each possible transport format until valid CRC bits are found. In the UMTS standard, which accommodates 16 possible transport formats, one would expect the process to have to be repeated eight times on average before finding the valid CRC bits.
This iterative process obviously consumes time, processing resources and battery life. Because it is inherently slow, BTFD is only used with relatively low rate channels (typically voice channels). In the case of high rate channels (typically data channels), TFIs must be transmitted (at the loss of some bandwidth).
Accordingly, what is needed in the art is a faster, less power consumptive way of carrying out BTFD in a receiver so high rate channels can be accommodated without requiring the transmission of bandwidth-reducing TFIs.